PV water heating system

ABSTRACT

A system for heating water includes a first tank, a solar panel, a control circuit, and a second tank for receiving water from the first tank. There is a first resistive heating element in the first tank and a second resistive heating element in the second tank. The control circuit couples power from the solar panel to the second resistive heating element unless the water in the second tank is at a preset maximum temperature, then the control circuit couples power to the first resistive heating element. If the water in the second tank is below a preset maximum temperature and above a preset minimum temperature, then the control circuit couples power from the solar panel to both the first heater element and the second heater element. If the temperature of the water in the second tank is below a preset minimum temperature, then the control circuit turns on conventional heating means in the second tank. Water is withdrawn from the second tank for use.

FIELD OF THE INVENTION

This invention relates to water heaters and, in particular, to a water heating system powered by electricity from a photovoltaic (PV) solar panel.

BACKGROUND OF THE INVENTION

The number of solar water heaters in use is less than one percent of the total number of water heaters. Of these, the vast majority are solar thermal water heaters. That is, solar energy directly heats water in a collector containing a plurality of pipes containing water. It is also known to convert solar energy to electricity and use the electricity to heat the water. This is PV water heating.

Solar thermal water heaters are not widely used because of problems of durability, difficulty of installation, weight, and relatively high initial cost. Durability includes problems with freezing, leakage, pump failure, and hard (mineral bearing) water. Installation has often proved difficult, requiring roof penetrations for the plumbing that transports water to and from solar collectors (arrays of pipes containing water that is heated by the sun).

U.S. Pat. No. 4,165,732 (Morin) discloses a solar voltaic preheat tank in combination with an electrically heated main tank.

U.S. Pat. No. 4,568,821 (Boe) discloses a water heating system having two tanks, a main tank heated by electricity or gas, and a solar thermal preheat tank. The main tank is normally off unless demand exceeds supply from the preheat tank. The main tank is bypassed when the preheat tank is used.

U.S. Pat. No. 4,948,948 (Lesage) discloses an electric water heater having plural heater elements. The elements dissipate different amounts of power from each other and are controlled by a timer for limiting peak demand on a power grid by selecting an element of less than maximum power.

U.S. Pat. No. 5,293,447 (Fanney et al.) discloses a solar voltaic water heater. A microprocessor controls a set of electrical relays that connect the photovoltaic module to several resistive heating elements in a manner that best matches the instantaneous operating characteristics of a photovoltaic module.

In general, a preheating tank is used in the prior art to reduce the load on a main tank by raising the temperature of the water from a cold water supply. While some attempt (Fanney et al.) is made to use photoelectric power efficiently, a point is reached in a single tank system at which available solar energy is not used because water temperature has reached a preset limit, e.g. 140° F. (60° C.).

Basically, the demand for hot water and the demand for solar energy do not match. A demand for large amounts of hot water, especially around sunrise, can significantly outstrip supply. The only solution is to increase the size of the tank, which increases rather than decreases the amount of energy needed. Increasing the size of the tank does not overcome the problem of limiting water temperature and introduces its own problem of having a larger mass of water to heat.

Water can absorb large amounts of energy with little change in temperature. Except for special cases where usage is limited or in hot climates where the cold water is already quite warm, PV solar systems are severely tested to provide enough energy at the right time to be the sole source of energy for heating water.

As used herein a “resistive” heating element does not mean that the element exhibits pure resistance, i.e. no reactive impedance. A resistive heating element is a device in which the flow of electric current through the device raises the temperature of the device itself.

In view of the foregoing, it is therefore an object of the invention to provide a solar photovoltaic heating system that works with conventional water heaters to provide large amounts of hot water with efficient use of energy.

Another object of the invention is to improve the efficiency of conventionally powered water heaters.

A further object of the invention is to provide a photovoltaic water heater that can be retrofitted to existing systems or incorporated into new systems.

Another object of the invention is to provide a photovoltaic water heating system that minimally relies on conventional heating.

SUMMARY OF THE INVENTION

The foregoing objects are achieved by this invention in which a system for heating water includes a first tank, a solar panel, a control circuit, and a second tank for receiving water from the first tank. There is a first resistive heating element in the first tank and a second resistive heating element in the second tank. The control circuit couples power from the solar panel to the second resistive heating element unless the water in the second tank is at a preset maximum temperature, then the control circuit couples power to the first resistive heating element. If the water in the second tank is below a preset maximum temperature and above a preset minimum temperature, then the control circuit couples power from the solar panel to both the first heater element and the second heater element. If the temperature of the water in the second tank is below a preset minimum temperature, then the control circuit turns on conventional heating means in the second tank. Water is withdrawn from the second tank for use.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a heater element constructed in accordance with a preferred embodiment of the invention;

FIG. 2 is an end view of the heater element illustrated in FIG. 1.

FIG. 3 is a schematic diagram of the electrical connections to a heater element constructed in accordance with a preferred embodiment of the invention;

FIG. 4 is a schematic diagram of the electrical connections to a heater element constructed in accordance with an alternative embodiment of the invention; and

FIG. 5 illustrates a photovoltaic water heating system constructed in accordance with a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional, expanded view of a heater element attached to the wall of a water heater. Heater element 10 includes plug 11 supporting a cantilever mounted resistive heating element 12. Plug 11 is sealed against bushing 14 and held in place by cap 15. Bushing 14 is welded or otherwise sealed to wall 13. Insulating member 16 contains connections for the resistive heating element and fits through aperture 17 in cap 15 for external access.

As illustrated in FIG. 2, insulating member 16 includes three terminals. In accordance with one aspect of the invention, there are at least two resistive heating elements, one for 110 volts AC and one for low voltage direct current. The amplitude of the low voltage depends upon the actual output voltage of a photovoltaic solar panel used with the system but is generally twenty four volts or higher.

The electrical connections are illustrated in FIG. 3. In a preferred embodiment of the invention the resistive heater elements share a common terminal for connection to what is called ground or common or neutral. One could use individual connections, four terminals, for the AC and DC resistive heating elements but this is not preferred. A three terminal connector is more compact. FIG. 4 illustrates an alternative embodiment of the invention in which the resistive heater elements share a common terminal but the low voltage resistive element is used for both AC and DC.

As illustrated in FIG. 1, resistive element 12 includes resistive material 31 inside sleeve 32. This prevents water from reacting with the resistive material, particularly when the material is hot. Other forms of heater element can be used instead. It is a detail that does not affect the invention. What matters is that the element match the applied voltage to produce the intended dissipation. For example, at 220 volts and 4500 watts, a resistive element has a resistance of approximately 10.8 Ω. At 24 volts and 1000 watts, a resistive element has a resistance of approximately 0.6 Ω.

In FIG. 5, solar panel 41 converts photons from sun 42 into an electric current. The electric current is applied to heater element 44 by control circuit 45. Heater element 44 converts the electric current to heat, thereby heating the water in tank 49. Solar panel 41 can be of any current or future design. The only constraint on the design is that panel 41 produce the appropriate voltage and current for the heater element 44.

In control circuit 45, semiconductor or solid state switching is preferred to relays, although either can be used for varying the power supplied to heater element 44. Heater element 44 includes one or more electrodes. If plural electrodes are used, then control circuit 45 can switch the electrodes in series or parallel combinations to provide the desired level of heating. Alternatively, pulsed direct current and switched electrodes can be used together to control the amount of power dissipated within tank 49.

Control circuit 45 also provides feedback to microprocessor 46, e.g. power level to heater element 44, voltage and current data on solar panel 41. Microprocessor 46 is preferably a single microcontroller chip having all necessary input/output (I/O), analog to digital (A/D) conversion, timing (including clock/calendar functions), and logic on a single chip. Alternately, separate devices for I/O, computation, conversion, and timing can be used.

Microprocessor 46 has several sense inputs, including photocell 51, thermocouple 53, and thermocouple 55. Photocell 51 provides a signal representing the level of available sunlight. Thermocouple 53 produces a signal representing the temperature of the water at the bottom of tank 49. Thermocouple 55 produces a signal representing the temperature of the water at the top of tank 49. Other transducers could be used as well; e.g. to sense whether or not tank 49 contains water, at least above the height of heater element 44, and to sense the flow of water to and from the tank. Water flow can be sensed indirectly as rate of change of temperature.

Water heater 61 has an inlet coupled to the output of tank 49 by pipe 62. The cold water entering inlet 57 mixes with the water stored in tank 49, cooling the stored water. Similarly, the warmed water entering tank 61 from pipe 62 mixes with the water in tank 61, transferring heat to the water in tank 61. Thus, even if the water in tank 49 be above 140° F., the water is isolated from a user and is cooled by mixing in tank 61.

In accordance with one aspect of the invention, microprocessor 46 controls heating in such a way as to minimize use of conventional power. The water in tank 61 can be heated by electricity or gas fired burner. If heated by electricity, the AC/DC heater element illustrated in FIG. 1 is substituted for an existing AC heater element. As illustrated in FIG. 5, heater element 71 is an AC/DC heater element.

Use of conventional power is minimized by using only photovoltaic energy when possible and by storing photovoltaic energy when it is not needed by the second tank as heated water in the first tank. Thus, for example, when thermocouples 73 and 74 indicate that the water in tank 61 is below some preset minimum, e.g. 110° F., then substantially all the power from solar panel 41 is dissipated in heater element 71. If the water in tank 61 is above the preset minimum, then some of the power from solar panel 41 is diverted to heater element 44. If the water in tank 61 is at maximum temperature, e.g. 120° F., then substantially all the power from solar panel 41 is dissipated in heater element 44. The temperature of the water in tank 49 is permitted to rise above the maximum temperature of the water in tank 61.

Depending upon the size of panel 41, the storage capacity of tank 49, and other circumstances, water in tank 49 could be heated above 160° F., a preferred upper limit. Thermocouples 53 and 54 provide feedback for limiting applied power, should limiting become necessary. It is also preferred that the water in tank 49 not be heated above 180° F. to avoid boiling the water on the surface of heater element 44. Building codes may also restrict the maximum temperature of the water in tank 49.

Only when it is sensed that the water in both tanks is going below some lower limit, e.g. 100° F., is conventional power used to augment solar power. Before conventional power is used, the temperature of the water in tank 61 must drop despite the application of solar power, if any is available, to heater element 71. In this way, conventional power becomes the source of last resort for heating water.

If tank 61 were part of a gas fired water heater, i.e. if it included gas burner 63, then heater element 71 could include only a DC element. The operation is otherwise the same. Water temperature determines which tank receives solar energy and tank 61 has precedence until it reaches maximum temperature.

The invention thus provides a solar photovoltaic heating system that works with conventional water heaters to provide large amounts of hot water with efficient use of energy and improves the efficiency of conventionally powered water heaters by minimally relying on conventional power. An AC/DC heater element can be retrofitted to existing systems or incorporated into new systems.

Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, in a minimal implementation of the invention, control circuit 15 could include one or more thermally sensitive switches or electrically controlled relays. A mechanical implementation of the invention is not preferred for reasons of reliability and the desire to provide more precise control. That is, semiconductor switches are preferred. The embodiments of FIG. 3 and FIG. 4 can be combined by “floating” the three terminals. Thus, one could have two power levels for AC; a lower power level including the DC resistive element and a higher power level including only the AC resistive element. This is not preferred because a switched ground connection is not preferred. An AC/DC heater element could be used in a gas fired water heater. 

1. In a system for heating water including a first tank, a solar panel, a control circuit, and a second tank for receiving water from the first tank, the improvement comprising: a first resistive heating element in the first tank; a second resistive heating element in the second tank; wherein the control circuit couples power from the solar panel to the second resistive heating element unless the water in the second tank is at a preset maximum temperature, then the control circuit couples power to the first resistive heating element; and water is withdrawn from the system through the second tank.
 2. The system for heating water as set forth in claim 1 wherein the control circuit couples power from the solar panel to the first heater element and the second heater element when the water in the second tank is below a preset maximum temperature and above a preset minimum temperature.
 3. The system for heating water as set forth in claim 1 wherein the second tank includes conventional water heating means.
 4. The system for heating water as set forth in claim 3 wherein the control circuit turns on the conventional heating means when the temperature of the water in the second tank is below a preset minimum temperature.
 5. The system for heating water as set forth in claim 3 wherein the control circuit turns on the conventional heating means when the temperature of the water in the second tank is below a preset minimum temperature and power is being coupled to the second resistive heating element.
 6. The system for heating water as set forth in claim 3 wherein the conventional heating means is a resistive heater.
 7. The system for heating water as set forth in claim 6 wherein the resistive heater and the second resistive heating element are attached to a single plug and share a common connection.
 8. The system for heating water as set forth in claim 7 wherein the resistive heater conducts alternating current.
 9. The system for heating water as set forth in claim 7 wherein the resistive heater conducts alternating current and the second resistive heating element conducts only direct current.
 8. The system for heating water as set forth in claim 3 wherein the conventional heating means is a gas burner. 